Trace elements in magnetite from the Jõhvi magnetite-rich gneisses in northeastern Estonia: implications for the genesis and alteration of iron mineralisation

The study aimed to determine the origin of iron mineralisation in the crystalline basement beneath the Jõhvi magnetic anomaly through the trace element geochemistry of magnetite. Magnetite-bearing samples from mineralised intervals were examined petrographically, and the trace element compositions of magnetite grains were characterised using scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry. The results show that magnetite trace element compositions cluster into distinct groups depending on the host rock. The Jõhvi magnetite ore most likely formed as a banded iron formation deposit. However, its trace element signature has been significantly altered by later metamorphic and hydrothermal processes. These results highlight that magnetite trace element analysis alone cannot reliably determine the origin of iron ore deposits that have undergone strong alteration.